This application is based on and hereby claims priority to German Application No. 101 57 080.5 filed on Nov. 21, 2001, the contents of which are hereby incorporated by reference.
The invention relates to a cryostat having a connecting branch which is connected to a cooling chamber and is open on the end side, for example according to DE 39 24 579 A1.
Cryostats of this type are known and are used wherever an object has to be cooled to a very low temperature. Liquid nitrogen having a temperature of 77 K or liquid helium having a temperature of 4.3 K are usually used as coolant, which is provided in a coolant chamber of the cryostat. A cryostat is used, for example, in a magnetic resonance investigation device primarily used for medical purposes (cf., for example, DE 39 24 579 A1, EP 0 587 423 B1 or EP 0 736 778 B1), or else in investigation devices for analytical purposes in the chemistry field (cf., for example, U.S. Pat. No. 4,291,541 A). When a cryostat is used in a magnetic resonance tomograph, the cryostat is used for cooling the superconductive magnet used for generating the basic field. The cryostat in question has an open connecting branch, i.e. it is an open system in which the coolant chamber containing the liquid coolant is connected to the environment. The liquid coolant does not rapidly volatilize via the open connecting branch because boiling equilibrium is set, and the supply of heat and energy to the coolant via the connecting branch is relatively small, so that very little coolant evaporates. Customary maintenance cycles within which coolant has to be topped up are approximately a year in the case of known magnetic resonance tomographs.
In the case of such a cryostat of a magnetic resonance tomograph, the connecting branch has a number of properties. Firstly, the first filling of the coolant chamber with coolant and topping up of the coolant can take place via the cryostat. Secondly, evaporating coolant can volatilize via the cryostat, the coolant having to volatilize in the case of an open system in order to avoid the internal pressure in the coolant chamber rising to an impermissibly high level. Moreover, the connecting branch is also used, if appropriate, for accommodating an electrode which is connected to the superconductive magnet when starting up the system. Via this electrode and a second electrode, which is likewise connected to the superconductive magnet, a current is guided over the superconductive magnet and, after reaching the transition temperature and with the magnet sufficiently cooled, is guided in a loss-free manner in the magnet, after which the two electrodes are separated from the superconductive magnet.
In this case, essentially three requirements have to be fulfilled by the connecting branch. Firstly, when accommodating an electrode it is to heat up as little as possible in order to avoid an impermissible high transport of heat taking place in the direction of the coolant chamber via the connecting branch or via the shield insulating the coolant chamber to the outside. Furthermore, small transport of heat from the environment into the interior of the cryostat during operation is to take place via the connecting branch. Finally, a pressure loss which is as small as possible has to be provided when volatilizing coolant flows through the connecting branch, for example in the event of a quench. In the case of a quench, the superconductive magnet becomes impermissibly hot at one point and transfers into the standard conductive state, which is associated with local heating which spreads and results, in the worst case, in the entire superconductive magnet transferring into the standard conductive state. Above all, the transport of heat into the interior of the cryostat via the connecting branch has a great effect on the duration of the maintenance cycle. The lower the heat input, the longer can the maintenance cycles be, which has a significant effect on the competitiveness of the product.
One aspect of the invention is therefore based on the problem of specifying a cryostat having a reduced heat input via the connecting branch.
In order to solve this problem, one aspect of the invention makes provision, in the case of a cryostat of the type mentioned at the beginning, for raised parts and/or depressions increasing the wall surface to be provided on at least part of the inner wall of the connecting branch.
One aspect of the invention is distinguished in that the wall surface of the connecting-branch inner wall is increased in a specific manner by raised parts and/or depressions. This is based on the finding that the heat input into the cryostat via the connecting branch is summarized as follows:
Qtotal=Qradiation+Qconductionxe2x88x92xcex1xcex94TA, where:
Qtotal=total heat input
Qradiation=heat input by heat radiation
Qconduction=heat input by heat conduction
xcex1=heat transfer coefficient
xcex94T=temperature difference between inner wall temperature and temperature of the volatilizing coolant
A=surface of the inner wall
As is apparent from the above equation, the heat radiation, which is passed into the cryostat through the connecting branch, and the heat conduction, which is input by the connecting branch or the connecting-branch material itself, increases the energy input. In contrast, it is reduced by the heat transfer from the wall to the discharge flow of the evaporating coolant. As described, the system is an open one in which some cooling medium evaporates, in albeit small quantities. The cooling medium brushes past the inner wall of the connecting branch and absorbs heat there, the quantity of heat absorbed being dependent on the heat transfer coefficient, the given temperature difference and the surface brushed over by outgoing coolant vapor.
One aspect of the invention takes this as the starting point by consciously enlarging the wall surface via the raised parts or depressions. The larger wall surface consequently increases the proportional quantity of heat which reduces the energy input and removed via the coolant discharge flow, with the result that, all in all, a reduction in the entire heat input and of the entire heat balance of the cryostat is produced.
If appropriate, at least one elongate, preferably centrally arranged component can extend in the axial direction in the space surrounded by the connecting branch. If this component is formed of metallic material, heat input into the interior of the cryostat can also take place via this component. Consequently, it is expedient if, in the case of a cryostat in which an elongate component preferably used as an electrode is provided in the connecting branch, raised parts and/or depressions increasing the outer wall surface are also provided on the outer wall of the component. This is because the coolant vapor also flows past this component and can thus absorb heat from this component and conduct it away to the outside.
The raised parts and/or depressions are expediently arranged running in the flow direction, so that, on the one hand, a flow profile which is as uniform and homogeneous as possible is produced, and, on the other hand, the pressure loss between the interior of the cryostat or the coolant chamber and the environment (if appropriate, also a discharge chamber), which adjoins the connecting branch, is as small as possible via the connecting branch.
The raised parts themselves can protrude in the form of ribs or plates from the wall surface. For example, these rib- or plate-like raised parts can be designed as narrow sheet metal sections which are arranged on the inner wall of the connecting branch. They are, expediently as is the connecting branch, made from stainless steel which inherently has low heat conduction at the extremely low temperatures.
It is particularly expedient if the raised parts are arranged at a distance from one another in the direction toward the coolant chamber. That is to say, the raised parts are, as it were, interrupted so as to avoid unintentionally increasing the heat conduction into the cryostat. If, for example, the abovementioned stainless steel sheet metal sections are used, then a plurality of short sheet metal sections can be arranged following one another and spaced apart from one another by a short distance in a symmetrical or asymmetrical distribution over the length of the connecting branch.
In addition to the described rib- or plate-like raised parts, for example in the form of sheet metal sections, it is, of course, also conceivable to produce the cavities and, resulting in between therefrom, the raised parts by corresponding grooves. Any type of texturing by removing material or joining on additional elements which lead to an increase in surface, is conceivable.
In addition to this described embodiment, in which specific texturing measures, by joining on elements or by specific removal of material, are thus used, an alternative makes provision for the wall surface to be rough or textured in order to produce the raised parts and/or depressions or further raised parts and/or depressions. This roughening or texturing can take place, for example, by specific etching of the surface or by the application of a coating having a surface which is rough, textured or enlarged in the manner of a relief. Of course, this measure may also be used in addition to the measures described at the beginning.
In addition to the cryostat itself, one aspect of the invention relates to a magnetic resonance device comprising a cryostat of the type described as part of this device.